A problem with known antennas that operate in the frequency range of 40 MHz to 860 MHz, the range that includes UHF, VHF and FM reception, is that over at least a portion of this range they are not good receivers.
Typically, commercially available antennas that cover this range are of the frequency-dependent type, which includes, among others, monopole and dipole antennas. The most commonly used frequency-dependent antennas for VHF and FM reception are half-wave dipole antennas, commonly referred to as rabbit-ear antennas.
Frequency-dependent antennas operate over a limited frequency range. The antenna output and other parameters vary significantly as a function of frequency, so as to make it necessary to adjust the antenna in some manner at each frequency of interest to cover a broader range of frequencies. For example, a half-wave dipole antenna may be fully extended to receive low-frequency transmission (e.g., channel 2 television), and may be progressively shortened to receive higher frequencies/channels. Additionally, the antenna may need rotation about its vertical axis to ensure that the beam peak points in the general direction of signal transmission.
Consequently, frequency-dependent antennas need frequent adjustment as the frequency intended to be received varies. Users often ignore this need, which contributes to sub-optimal performance. Prior attempts to eliminate the need for frequent adjustment have resulted in an abundance of tuning requirements that have complicated operation to the degree where it is not only inconvenient to a user, but also nearly impossible to actually reach an optimum level of performance.
An additional problem with frequency-dependent antennas is that the gain is relatively low, on the order of 1 DB. The gain is often improved (i.e. signal reception is strengthened) through active signal amplification at the antenna output, but at the expense of an increase in system noise, which always occurs when pre-amplification is employed. This creates an additional need for DC power. Such an active system (i.e., one requiring DC power to operate) is more costly, more complicated, and more likely to break down.
Frequency-independent antennas, by contrast, require little or no adjustment throughout the entire range over which they operate because the antenna output and other parameters do not vary significantly as a function of frequency over the specified bandwidth of the antenna. Such antennas are especially attractive for broadband applications in instances where active signal amplification is not required. However, their limitation is that they must be very large to receive low-frequency transmissions, severely limiting their usefulness in a home environment. A relatively small stand-alone frequency-independent antenna is not capable of effectively receiving signals in the low-frequency range.
A spiral antenna, for instance, is a wellknown type of frequency-independent, broadband antenna that requires no tuning over a wide range of frequencies. Spiral antennas are typically used in military applications which, by their very nature, do not allow for frequent adjustment of antenna structures. For example, spiral antennas are often mounted in the belly of aircrafts for use in situations in which the direction of signal transmission, the particular signal frequency, and the time of signal receipt are not known, and the position of the antenna is not, and indeed cannot be, adjusted. As a result, the received signals contain a large amount of noise. Through the use of sophisticated and expensive electronic processing units, a large portion of the noise can be removed, and, thus, the direction and frequency of the signals can be determined.
An Archimedes spiral antenna comprises at least one radiating element formed into a spiral in accordance with a predetermined mathematical formula. If the antenna comprises two or more radiating elements, the radiating elements are typically interleaved.
The rate of growth of a conductor is the rate at which the radiating elements spiral outwardly. The number of conductors and their rate of growth have a direct relationship to the frequency range to be covered by the antenna. In general, a signal is received at a portion of the spiral antenna having a circumference equal to the wavelength of the signal. The low-frequency limit of a spiral antenna is defined as the frequency of a signal with a wavelength equal to the largest circumference of the spiral antenna. Therefore, to receive the long wavelengths of low-frequency transmission, the spiral must be quite large. For example, a spiral antenna used to receive channel 2 television transmissions would have to have a diameter of approximately 6 feet, and a circumference of approximately 19 feet. For obvious reasons, this size factor severely limits the usefulness of spiral antennas in a horne environment. Moreover, UHF/VHF/FM antennas are typically inexpensive structures that cannot afford the use of sophisticated signal processing equipment.
A need therefore exists for a relatively inexpensive antenna that covers a broad range of frequencies with sufficient signal reception throughout the broad frequency range while having a streamline construction and providing ease of use.